The invention pertains to a solid-state imaging sensor used for producing signals which represent the intensity of a light pattern. Such a solid-state imaging sensor may be used, for example, as a detector (target) in a television camera or electronic still camera.
Solid-state imaging sensors of the general type to which the invention pertains are composed of a matrix of pixels (picture elements) arranged in rows and columns. Each of the pixels receives light from a corresponding portion of the picture or image projected onto the sensor, and produces in response thereto an electrical signal representing the intensity of that portion.
As depicted in FIG. 1, one prior art approach to sensor construction was to form, on a semiconductor substrate 10, pixels 11 in a regular rectangular pattern of linear rows and linear columns on a semiconductor substrate 10. Horizontally extending electrodes and vertically extending electrodes (not shown in the view of FIG. 1) are provided, respectively, for actuating in sequence the various rows of pixels 11 and for receiving the signals read out from the rows of pixels. As is well known to those of ordinary skill in the art, the horizontally extending electrodes are connected to outputs of what is termed a "vertical shift register" for actuation.
Although the arrangement of FIG. 1 was acceptable for some applications, nevertheless, it suffered from some significant drawbacks. First, the resolution of the sensor, as measured by the number of pixels per unit distance in the horizontal direction, was limited. Secondly, because of the linear arrangement in both the row and column directions, moire interference could occur when certain patterns were being sensed.
In an attempt to overcome these drawbacks, a pixel arrangement such as that illustrated in FIG. 2A has been proposed. In this arrangement, the pixels 11 are formed on the semiconductor substrate 10 in a staggered arrangement, that is, alternate rows of the pixels 11 are offset from one another in the row direction.
FIG. 2B is an enlarged top schematic view of the imaging sensor of FIG. 2A, and FIG. 2C is a cross-sectional view taken along a line A--A' in FIG. 2B. With reference now to FIG. 2B, the light sensitive area of each of the pixels 11 is defined by the area of a respective source diffusion 15 which forms both an active layer of a photodiode and the source of a switching FET device used for connecting the output of the photodiode to a respective one of the vertically extending lines. One end of each source diffusion 15 extends up to the edge of a corresponding horizontally extending electrode 18, and a drain diffusion 16 is formed on the opposite side of the horizontally extending electrode 18. The horizontally extending electrode is separated from the surface of the semiconductor substrate by a thin oxide layer so as to form a gate region 19 between each source diffusion 15 and drain diffusion 16. The drain diffusions 16 are connected to respective vertically extending lines 24 (see FIG. 2C, omitted in FIG. 2B for clarity).
With specific reference to FIGS. 2C, each source diffusion 15 may be an N.sup.+ -type diffusion formed in a P well 26. A P.sup.+ region 25 is formed under the N.sup.+ -type diffusion 15 on an N-type substrate 27. In this manner, the source diffusion 15 forms a photodiode with the P.sup.+ region 25 and P-type well 26. Each horizontally extending electrode 18 is insulated from the vertically extending electrodes 24 by an insulating oxide layer 20. If the imaging sensor is to be a color imaging sensor, a color filter 23 is provided over each source diffusion 15 with the various color filters of the array being provided in a pattern appropriate for color imaging. The color filters 23 are embedded in protective layer 22.
The arrangements of FIGS. 2A-2C is advantageous over the one depicted in FIG. 1 in that the resolution of the sensor is improved and moire fringing effects are reduced. However, the sensor of FIGS. 2A-2C is still not fully acceptable for many applications because its sensitivity is not sufficient.
The sensitivity of a pixel can be determined by dividing its photosensitive area (the area defined by the source diffusion 15 in the sensor of FIGS. 2A-2C) by the total area of the pixel. Thus, it can be seen that by increasing the number of pixels per unit distance in the row direction in the arrangement of FIGS. 2A-2C in order to improve the resolution of the sensor, for a given minimum photolithographic definition, for example, a given minimum width of one of the horizontally or vertically extending electrodes, the sensitivity of the sensor is lowered.
Accordingly, it is an object of the present invention to provide a solid-state imaging sensor of the same general type described above, but in which both the resolution and sensitivity are improved over prior art approaches.